Nuclear waste heat generating system

Abstract

A nuclear waste heat generating system (10) comprises nuclear waste storage site (12) containing one or more storage containers (14) comprising radioactive nuclear waste. The heat pump system (16) comprises a closed working fluid circuit (26) filled with a working fluid, the working fluid circuit including an inflow pipe (22) for supplying the working fluid from a heat sink (18) of the heat pump system (16) to a heat exchanger (20) of the heat pump system (16), and an outflow pipe (24) for returning heated working fluid from the heat exchanger (20) to the heat sink (18). The heat exchanger (20) is thermally coupled to the nuclear waste storage site (12) such that the heat exchanger (20) is heated by decay heat produced by the one or more storage containers (14). Further, a use of one or more storage containers comprising radioactive nuclear waste as a heat source is described.

Description

[0001] Transmutex SA

[0002] Linser Zeichen: T 15662 WO

[0003] WS / TH

[0004] Nuclear waste heat generating system

[0005] The invention relates to a nuclear waste heat generating system and a use of radioactive nuclear waste as a heat source.

[0006] The increasing need for energy is becoming a critical problem and solutions to this problem need to take into account sustainable energy sources and recycling of existing supplies helping to preserve natural resources and to reduce environmental degradation.

[0007] Renewable energy sources relying on solar, wind and hydropower are intermittent and require vast interference in the landscape such that renewable energy sources are not an optimal solution in many regions.

[0008] Another option is geothermal energy which provides reliable and consistent energy supply based on geothermal heat pumps which utilize the relatively constant temperatures found below the Earth’s surface to provide heating, cooling and / or hot water for facilities. However, the availability of geothermal energy is region-specific and identifying viable geothermal sites requires extensive geological surveys and exploration, which can be expensive and timeconsuming.

[0009] A further option is nuclear energy which has the advantage of a consistent power supply. However, the nuclear waste obtained during operation of a nuclear power plant is considered a burden of this type of energy generation. Nuclear waste is currently stored in interim storage facilities, and is planned to be transferred to deep geological repository sites for permanent storage. Nuclear waste, particularly spent nuclear fuel and other highly radioactive waste, generates a significant amount of heat due to the radioactive decay of the radioactive isotopes contained therein. This heat generation is a critical factor in the design and management of any type of storage site for nuclear waste.

[0010] E.g., in the so-called ZWILAG interim storage site in Switzerland, the heat generated by the casks comprising nuclear waste stored and placed within a storage hall is dissipated by natural circulation of air through openings in the storage hall to limit the temperature within the storage hill to acceptable levels. In an article by Wigeland et al. (“Separations and Transmutation criteria to improve utilization of a geologic repository , Nuclear Technology, Vol. 154 (2006), pp. 95 - 106, doi: 10.13182 / NT06-3), the influence of heat generation in the design of a planned deep geological repository at Yucca Mountain, United States, is investigated. For controlling the temperature increase within emplacement drifts containing the nuclear waste to acceptable levels, it is proposed to implement a forced ventilation through the respective drifts over a period of at least 50 years. Such a forced ventilation is thus connected with a considerable effort and high costs.

[0011] It is the object of the invention to provide a system for efficiently generating heat and / or electricity. The system should especially provide for a consistent energy supply.

[0012] The object of the invention is solved by a nuclear waste heat generating system comprising a nuclear waste storage site containing one or more storage containers comprising radioactive nuclear waste, and a heat pump system. The heat pump system comprises a closed working fluid circuit including an inflow pipe for supplying the working fluid from a heat sink of the heat pump system to a heat exchanger of the heat pump system, and an outflow pipe for returning heated working fluid from the heat exchanger to the heat sink. The heat exchanger is thermally coupled to the nuclear waste storage site such that the heat exchanger is heated by decay heat produced by the one or more storage containers.

[0013] The decay heat produced by the one or more storage containers is based on the radioactive decay of the radioactive nuclear waste stored therein.

[0014] The working fluid flowing through the heat exchanger is heated due to the heat received by the heat exchanger from the nuclear waste storage site. That is, the working fluid is heated while flowing through the heat exchanger for becoming the heated working fluid and is cooled again within the heat sink after flowing back to the heat sink through the outflow pipe.

[0015] The invention is based on the idea to actively use the decay heat which is produced by radioactive nuclear waste due to radioactive decay for energy generation, especially for heat generation and optionally power generation. In this way, nuclear waste, which has been up to now being considered as unwanted by-products from nuclear facilities only, is put to a second use as energy source in the heat generating system.

[0016] The heat is removed from the one or more storage containers by means of a heat exchanger being part of a heat pump system. Heat pump systems are very efficient due to low energy losses within the system and can be installed both in storage sites above ground, i.e. on the Earth’s surface, and within geological formations by drilling shafts in which the necessary inflow and outflow pipes are placed. The coefficient of performance (COP) of a heat pump system is dependent on the temperature difference between the heat source and the heat sink used within the system. Due to the high temperatures generated by the radioactive decay of the radioactive nuclear waste, a high temperature can be generated within the nuclear waste storage site such that the temperature difference between the nuclear waste storage site as heat source and the heat sink is improved compared to conventional geothermal energy facilities.

[0017] The decay heat per time unit generated by nuclear waste decreases over time such that directly after placing the one or more storage containers in the nuclear waste storage site, the temperature difference of the heat pump system is the highest, resulting in an especially efficient operation of the heat pump system while providing for cooling of the one or more storage containers when needed the most to avoid an excessive temperature rise of the one or more storage containers. Further, even with comparatively short-lived radioisotopes typically encompassed in radioactive nuclear waste, said nuclear waste will constantly generate heat for an extended period of time, typically at least a hundred years or more, such that a reliable and consistent energy generation becomes possible.

[0018] At the same time, the heat removed by the heat pump system from the nuclear waste storage site effectively cools the one or more storage containers such to ensure that the temperature increase within the nuclear waste storage site is limited to acceptable levels. Especially, further measures for extracting heat from the nuclear waste storage site like an active ventilation or the release of heated air can be omitted or at least reduced, thereby lowering the costs of operating the nuclear waste storage site. The thermal coupling between the heat exchanger and the nuclear waste storage site can be direct or indirect via a heat-transporting device.

[0019] E.g., the heat exchanger can be placed within the nuclear waste storage site to minimize heat losses during heat transport from the nuclear waste storage site to the heat exchanger.

[0020] In one variant, the heat exchanger is placed within the nuclear waste storage site spaced apart from the one or more storage containers and is configured to exchange heat with air or a gas filling the nuclear waste storage site. Thus, the decay heat generated by the one or more storage containers is transferred to the surrounding air or gas which then flows towards the heat exchanger and further transfers the heat to the heat exchanger.

[0021] The heat exchanger can also be arranged within a flow path of exhaust air or exhaust gas flowing away from the nuclear waste storage site toward the heat exchanger.

[0022] In another variant, the heat exchanger is at least partially integrated in at least one of the one or more storage containers. In this way, an especially good heat transfer from the one or more storage containers to the heat exchanger is realized, resulting in especially low heat losses and an efficient cooling of the one or more storage containers.

[0023] The heat exchanger can be integrated in more than one of the one or more storage containers, especially in all of the storage containers.

[0024] E.g., the heat exchanger can be welded or bolted to the at least one of the one or more storage containers. This allows to integrate the heat exchanger with the respective storage container after the storage container is placed within the nuclear waste storage site and provides for a secure and reliable connection between the heat exchanger and the storage container.

[0025] The nuclear waste storage site can be an interim storage facility. Interim storage facilities are used to store radioactive nuclear waste until the radioactive nuclear waste can be transported for permanent storage to a final storage site, typically a deep geological repository. Interim storage facilities typically are built above ground level and define a confined space or hall in which the one or more storage containers are placed, thereby heating the air or gas within the confined space or hall based on the decay heat of the radioactive nuclear waste. An example of an interim storage facility is the so-called ZWILAG cask storage hall in Wurenlingen, Switzerland.

[0026] Alternatively, the nuclear waste storage site can be a geological repository site like a deep geological repository. A geological repository site denotes an underground facility formed in a geological formation and is typically intended as permanent storage for nuclear waste. E.g., the geological repository site can be a tunnel or drift formed within the geological formation.

[0027] The storage capacity of a given nuclear waste storage site, i.e. the amount of storage containers comprising radioactive nuclear waste placed within the site, is dependent on the heat management of the decay heat produced by the nuclear waste. By extracting the decay heat with the heat pump system, the storage capacity of a given nuclear waste storage site can be increased, as the maximum temperature increase per time unit within the nuclear waste storage site can be limited based on controlling the operation of heat pump system, e.g. dependent on the flow rate of the working fluid within the closed working fluid circuit and / or the temperature of the working fluid in the inflow pipe.

[0028] The radioactive nuclear waste can be transmuted and / or reprocessed radioactive nuclear waste being free from radioactive isotopes with a half-life of 1,000 years or more. The term “being free form” means being free from the respective species other than unavoidable impurities. The transmuted and / or reprocessed radioactive nuclear waste emits for the first couple of hundred years more decay heat per unit volume than unprocessed radioactive nuclear waste because of the higher content of radioisotopes having a shorter half-time but therefore higher radioactive activity. Thus, the efficiency of the heat pump system is further improved, while at the same time the overall safety of the nuclear waste heat generating system is increased, as storage containers used for storing radioactive nuclear waste can be reliably expected to withstand storage over such timescales without damages and stay waterproof. Thus, the risk of breakages of the storage containers over the reduced lifetime of the radioactive nuclear waste is lowered. E.g., the transmuted radioactive waste can be a radioactive nuclear waste generated from nuclear fuel for a thorium fuel cycle.

[0029] The transmuted and / or reprocessed nuclear waste can be a nuclear waste having a radioactivity which equals natural radioactivity after less than 1,000 years, especially after 150 to less than 1,000 years or after 300 to less than 1,000 years.

[0030] To extract the heat from the heated working fluid flowing through the outlet pipe from the heat exchanger to the heat sink, the heat sink can comprise a secondary heat exchanger and / or an electric generator.

[0031] The secondary heat exchanger can be connected to a heat distribution system, e.g. a heat distribution system configured for transferring heat to a building, a facility and / or to infrastructure entities like a district heating network.

[0032] The electric generator allows to convert the heat extracted from the nuclear waste storage site into electrical power which can then be used to power the further components of the heat pump system and / or can be fed into an electrical grid.

[0033] The object of the invention is further solved by a use of one or more storage containers comprising radioactive nuclear waste as a heat source in a nuclear waste heat generating system according to the descriptions above.

[0034] The features and advantages described above for the nuclear waste heat generating system according to the inventions apply to the use according to the invention, too, and vice versa and it is referred to the explanations given above.

[0035] By using the radioactive nuclear waste as energy source, the nuclear waste is put to a second use instead of just taking care of its storage without any beneficial effect.

[0036] Further properties and advantages of the invention will become more apparent from the following description of exemplary embodiments of the invention, which are not to be understood as limiting, and from the accompanying drawings. In the drawings: - Fig. 1 shows a schematic depiction of a first embodiment of a nuclear waste heat generating system according to the invention, and

[0037] - Fig. 2 shows a schematic depiction of a second embodiment of a nuclear waste heat generating system according to the invention.

[0038] Fig. 1 schematically shows a first embodiment of a nuclear waste heat generating system 10 according to the invention.

[0039] The nuclear waste heat generating system 10 comprises a nuclear waste storage site 12 in which several storage containers 14 comprising radioactive nuclear waste are placed.

[0040] The type of nuclear waste storage site 12 is not specifically limited, as long as it is suited for storing the storage containers 14. E.g., the nuclear waste storage site 12 can be a storage hall of an interim storage facility in which the storage containers 14 are placed. Alternatively, the nuclear waste storage site 12 can be a geological repository site comprising one or more tunnels or drifts in which the storage containers 14 are placed for permanent storage.

[0041] The radioactive nuclear waste contained in the storage containers 14 are radioactively decaying, thereby producing decay heat. The decay heat results in a temperature increase of the storage containers 14 themselves and of air or a gas filling the interior space of the nuclear waste storage site 12.

[0042] The temperature increase limits the number of storage containers containing a given type of radioactive nuclear waste within the nuclear waste storage site 12 such to avoid that the temperature within the nuclear waste storage site 12 exceeds a pre-defined temperature limit.

[0043] E.g., in case of a geological repository site, the pre-defined temperature limit can be chosen such to ensure that the geological formations surrounding the nuclear waste storage site 12 remain stable. It is also possible that the predefined temperature limit is chosen such that no steam formation can occur within the nuclear waste storage site 12 and / or the storage containers 14.

[0044] In Fig. 1, a total of four storage containers 14 are shown for illustrative purposes. Of course, the number of storage containers 14 can be different than what is shown in the Figures. Typically, several tens to several hundreds of storage containers 14 are stored within a given nuclear waste storage site 12.

[0045] The nuclear waste heat generating system 10 further comprises a heat pump system 16.

[0046] The heat pump system 16 comprises a heat sink 18, a heat exchanger 20, an inflow pipe 22 fluidly connecting the heat sink 18 with an inlet of the heat exchanger 20, and an outflow pipe 24 connecting an outlet of the heat exchanger 20 with the heat sink 18.

[0047] The heat sink 18, the inflow pipe 22, the heat exchanger 20 and the outflow pipe 24 form a closed working fluid circuit 26, which is filled with a working fluid.

[0048] The working fluid is pumped by the heat pump system 16, e.g. by a (not explicitly shown) condenser of the heat pump system 16 located within the heat sink 18, from the heat sink 18 towards the heat exchanger 20, is heated up in the heat exchanger 20, and flows back from the heat exchanger 20 as heated working fluid to the heat sink 18, in which the heat taken up by the working fluid in the heat exchanger 20 is at least partially extracted such that the cooled working fluid can be directed again towards the heat exchanger 20.

[0049] The general construction of the heat pump system 16 for realizing the closed working fluid circuit 26 can be analogously to heat pump systems known in the art, e.g. of geothermal heat pumps.

[0050] Within the heat sink 18, the working fluid can be cooled by a secondary heat exchanger 28. In the shown embodiment, the secondary heat exchanger 28 is connected to an electric generator 30 in which the heat extracted from the working fluid is used for power generation. However, it is also possible that the heat extracted from the working fluid is directly used to transfer heat to a building, a facility or a district heating network.

[0051] As indicated by dashed lines in Fig. 1, the heat sink 18 can be arranged spaced apart from the nuclear waste storage site 12. E.g., in the case of a geological repository site being used as nuclear waste storage site 12, the inflow pipe 22 and the outflow pipe 24 can extend through a drilled shaft to above ground-level where the heat sink 18 is placed. However, it is also possible that the heat sink 18 is placed close by or directly adjacent to the nuclear waste storage site 12, especially in case the nuclear waste storage site 12 is an interim storage facility.

[0052] The heat exchanger 20 is placed within the nuclear waste storage site 12, but spaced apparat from the storage containers 14. That is, heat transfer from the storage containers to the heat exchanger 20 is mediated by the air or the gas filling the nuclear waste storage site 12. Such a solution is especially easy to integrate in a given nuclear waste storage site 12 and avoids direct contact between the heat exchanger 20 and the storage containers 14.

[0053] Fig. 2 shows a second embodiment of the nuclear waste heat generating system 10 according to the invention.

[0054] The second embodiment essentially corresponds to the first embodiment such that in the following only differences will be described. Same reference numerals denote the same or functionally same components and it is referred to the explanations given above.

[0055] In the second embodiment, the heat exchanger 20 is at least partially integrated in the storage containers 14. That is, the heat exchanger 20 is designed such to be in direct contact with the storage containers 14.

[0056] The heat exchanger 20 may be physically connected to the storage containers 14, e.g. welded or bolted to the storage containers 14, to ensure for a reliable and secure connection.

[0057] Due to the direct contact between the heat exchanger 20 and the storage containers 14, the heat transfer does not need to be mediated by the air or gas filling the nuclear waste storage site 12, resulting in a faster and even more efficient heat transfer. Further, the storage containers 14 are directly cooled by the heat exchanger, which can further limit the temperature of the storage container 14 and allows the temperature within the nuclear waste storage site 12 and possible surroundings like rock formations to be lower.

[0058] In Fig. 2, the heat exchanger 20 is integrated in all of the storage containers 14. Of course, it is also possible that the heat exchanger 20 is only integrated with a single one or a selected sub-set of the storage containers 14. In this case, the effort for integrating the heat exchanger 20 into the storage containers 14 can be lowered. The decay heat produced by the storage containers 14 in which the heat exchanger 20 is not integrated, are then again cooled by a heat transfer mechanism to the heat exchanger 20 mediated by the air or a gas filling the nuclear waste storage site 12 as described before.

[0059] Overall, the nuclear waste heat generating system 10 relies on the use of the storage containers 14 comprising radioactive nuclear waste as a heat source. This allows for a reliable and consistent heat and / or power supply, while at the same time the storage capacity of the nuclear waste storage site 12 can be improved and the radioactive nuclear waste can be put to a second use as energy source.

Claims

Claims1. A nuclear waste heat generating system (10) comprising a nuclear waste storage site (12) containing one or more storage containers (14) comprising radioactive nuclear waste, and a heat pump system (16) comprising a closed working fluid circuit (26) filled with a working fluid, the working fluid circuit including an inflow pipe (22) for supplying the working fluid from a heat sink (18) of the heat pump system (16) to a heat exchanger (20) of the heat pump system (16), and an outflow pipe (24) for returning heated working fluid from the heat exchanger (20) to the heat sink (18), wherein the heat exchanger (20) is thermally coupled to the nuclear waste storage site (12) such that the heat exchanger (20) is heated by decay heat produced by the one or more storage containers (14).

2. The nuclear waste heat generating system (10) according to claim 1 , wherein the heat exchanger (20) is placed within the nuclear waste storage site (12) spaced apart from the one or more storage containers (14) and is configured to exchange heat with air or a gas filling the nuclear waste storage site (12).

3. The nuclear waste heat generating system (10) according to claim 1 , wherein the heat exchanger (20) is at least partially integrated in at least one of the one or more storage containers (14).

4. The nuclear waste heat generating system (10) according to claim 3, wherein the heat exchanger (20) is welded or bolted to the at least one of the one or more storage containers (14).

5. The nuclear waste heat generating system (10) according to any of the preceding claims, wherein the nuclear waste storage site (12) is an interim storage facility.

6. The nuclear waste heat generating system (10) according to any of claims 1 to 4, wherein the nuclear waste storage site (12) is a geological repository site.

7. The nuclear waste heat generating system (10) according to any of the preceding claims, wherein the radioactive nuclear waste is transmuted and / or reprocessed radioactive nuclear waste being free from radioactive isotopes with a half-life of 1,000 years or more.

8. The nuclear waste heat generating system (10) according to any of the preceding claims, wherein the heat sink (18) comprises a secondary heat exchanger (28) and / or an electric generator (30).

9. A use of one or more storage containers (14) comprising radioactive nuclear waste as a heat source in a nuclear waste heat generating system (10) according to any of the preceding claims.

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